3.1 ¹⁰Be exposure dating

Based on standard sampling criteria (see Gosse and Philips, 2001; Ivy-Ochs and Kober, 2008), we collected 11 samples from morainic boulders and one bedrock sample for exposure dating. The geomorphological evaluation and the careful selection of every boulder are crucial for the reliability of the dating results. Often, boulders are not optimal, or only one boulder on a moraine fits the tight criteria for surface exposure dating (e.g., being large enough). Where enough suitable boulders are present, sampling several (∼3–5) boulders per moraine can increase the trustworthiness of the age attribution (Putkonen and Swanson, 2003; Ivy-Ochs and Kober, 2008). But relying on one boulder that meets all geomorphologic criteria can be more promising and effective than dating mediocre samples (Ivy-Ochs et al., 2007).

The rock material was obtained with a battery-powered saw, hammer and chisel. The lithology of the samples was gneiss and granite. The chemical isolation of the ¹⁰Be followed in-house standard procedures, mainly based on the work of Kohl and Nishiizumi (1992) and Ivy-Ochs et al. (1996). Initially the samples were crushed and sieved (0.25–0.6 mm). Then organic and calcareous contaminations were removed with aqua regia (a mixture of nitric and hydrochloric acid). After a 1 h treatment with 0.4 % hydrofluoric acid (HF) we used a froth floatation system to physically separate micas and feldspar components from the sample. To obtain pure quartz, the samples were treated with 4 % HF. Subsequently we added a ⁹Be carrier solution to the dried quartz and dissolved the samples with 40 % HF. Afterwards Be was isolated using ion exchange resins and selective pH precipitation techniques (von Blanckenburg et al., 1996). The ¹⁰Be ∕ ⁹Be ratios were measured with the TANDY mass spectrometer of the ETH Laboratory of Ion Beam Physics against the ¹⁰Be standard S2007N with a nominal value of ¹⁰Be ∕ ⁹Be = $\mathrm{28.1}\times {\mathrm{10}}^{-\mathrm{12}}$ (Kubik and Christl, 2010; Christl et al., 2013). S2007N has been calibrated to the ¹⁰Be standard ICN 01-5-1, which has a nominal ¹⁰Be ∕ ⁹Be value of $\mathrm{2.709}\times {\mathrm{10}}^{-\mathrm{11}}$ (Nishiizumi et al., 2007). Stated ¹⁰Be ∕ ⁹Be ratios include analytical uncertainties of the accelerator mass spectrometry (AMS) measurement (errors: 1σ level). The exposure ages were calculated from the blank-corrected measurements (blank value: $(\mathrm{0.007}\pm \mathrm{0.0021})\times {\mathrm{10}}^{-\mathrm{12}})$ using the online tool CRONUS-Earth (Balco et al., 2008) (http://hess.ess.washington.edu, last access: April 2018) with a ¹⁰Be half-life of 1.387±0.012 Myr (Chmeleff et al., 2010; Korschinek et al., 2010) and the northeastern North America (NENA) ¹⁰Be production rate of 3.87±0.19 ¹⁰Be atoms gram_qtz⁻¹ yr⁻¹ (Balco et al., 2009). The calculation of a local ¹⁰Be production rate at the Chironico landside in the southern Swiss Alps has shown that this production rate is very suitable for the Alps (Claude et al., 2014). Moreover it matches well with recently calculated global ¹⁰Be production rates (Heyman, 2014; Borchers et al., 2016). Exposure ages were scaled to the geographical location according to the scheme of Lal (1991) and Stone (2000) and corrected for the sample geometry. The effect of geometric shielding by surrounding mountains on the amount of cosmic radiation reaching each sample was calculated on the basis of field data with the online geometric-shielding calculator CRONUS (http://hess.ess.washington.edu, last access: April 2018). We applied an erosion rate of 1 mm kyr⁻¹ (André, 2002) to determine the exposure ages.

In contrast to boulder samples, exposure ages from bedrock samples are normally corrected for the shielding influence of snow. A snow correction was performed for the single bedrock sample Erstfeld Sackberg, in accordance with Gosse and Philips (2001), on the basis of a snow density of 0.3 g cm⁻³ and an attenuation length of 109 g cm⁻² (Zweck et al., 2013; Delunel et al., 2014). For the calculation we assumed a closed snow cover of 10 cm over 3 months. We base this assumption on data from the MeteoSwiss station in Altdorf (438 m a.s.l.). The sample location lies approximately 150 m higher. We therefore used slightly higher snow cover values. The snow cover throughout the Lateglacial and Holocene was most likely very variable. The snow correction factor (0.993) is therefore connected to considerable uncertainty. Because of the low snow cover at the sample altitude, the derived snow-corrected exposure age is only negligibly older than the uncorrected exposure age (ca. 100 a).

In the discussion we therefore use the exposure age without snow correction.

3.2 Radiocarbon dating of peat

The peat bog northeast of the Arnisee could only start to develop after the Reussgletscher had vanished from the area. Dating the onset of the bog formation therefore provides a minimum age for the ice retreat. We used avalanche probes to detect the thickest and potentially oldest point of the bog and subsequently retrieved cores with a handheld peat drill. For dating, we extracted peat fragments from the lowermost layer (1 cm), directly above the inorganic sediments (depth 475 cm) that were deposited prior to the bog formation. The peat fragments were cleaned using an acid–alkali–acid (AAA) treatment and heated under vacuum in quartz tubes with CuO (oxygen source). The tubes were then evacuated again, sealed and heated in the oven at 900 ^∘C to obtain CO₂. The CO₂ of the combusted sample was mixed with H₂ (1 : 2.5) and catalytically reduced over iron powder at 535 ^∘C to elemental carbon (graphite). After reduction, the mixture was pressed into a target and the carbon ratio was measured by AMS using the 0.2 MV radiocarbon dating facility (MICADAS) of the ETH Laboratory of Ion Beam Physics. The calendar age was calibrated using OxCal 4.3 (Bronk Ramsey, 2009), based on the IntCal 13 calibration curve (Reimer et al., 2013). The calibrated age is given in the 2σ range.

4.1 Schattdorfer Berge

The Schattdorfer Berge area is marked by a very interesting glacial geomorphology (Figs. 2 and 3): in the northern part a series of five right-lateral moraines (I–V) at altitudes between 1000 and 1300 m is preserved. Their position and direction indicate that the moraines were deposited by a more than 1000 m thick main valley glacier (Spillmann et al., 2011). Crystalline erratics undoubtedly prove that the Reussgletscher deposited these moraines, as the mountain range of the Hoch Fulen unit directly to the south is built of limestone (Labhart and Pfiffner, 2011). Corresponding terminal positions are most probably subaquatic glacigenic landforms found at the bottom of Lake Lucerne. A clear assignment to the suggested moraine stages in the lake (from distal to proximal: Meggen, Kehrsiten (Chrüztrichter), Nase (Vitznau) and Chindli (Gersau); Penck and Brückner, 1909; Hilbe et al., 2011) has to remain open at this point. To address the depositional ages, we took five rock samples from different moraines and ice margin positions at the Schattdorfer Berge site. The three samples Studenbergliwald 1–3 did not contain enough quartz to complete the chemical processing. The two samples Schattdorf 1 and 2 were dated to 13 020±820 a and 6150±470 a, respectively. Considering the stratigraphic position within the Reussgletscher system, both samples seem far too young, also given the fact that YD-aged moraines in the Reuss catchment have been described much further up-valley, i.e., at the entrance to the Göschenertal (Fig. 1) (Boxleitner et al., 2017). Considering the high and therefore potentially old sampling site of the sample Schattdorf 2, this boulder was most likely exhumed well after its deposition at the former ice margin. An explanation for the too-young age of the sample Schattdorf 1 is not that obvious. A degradation of the landform due to the construction of the small chapel on top of the moraines and hence a later exhumation of the sampled boulder is a possible but unverifiable reason for this result (Fig. 3). In the end, the timing of the deposition of these moraines remains unclear despite our dating efforts.

In the southern part of the Schattdorfer Berge a series of moraines (I–III), including a well-preserved terminal moraine (Figs. 2 and 3), witnesses repeated advances of a local glacier originating from the northern slopes of the Hoch Fulen and the Bälmeten. During the main valley glaciation, the Hoch Fulen–Bälmeten glacier was a tributary to the Reussgletscher. After a pronounced lowering of the ice surface in the Reusstal, this glacier re-advanced on a local scale, crosscut the lateral moraines of the former Reussgletscher and produced moraines that are, based on their stratigraphic position, indisputably younger than the ones right to the north. In addition to the clearly different morphostratigraphic positions, the two moraine generations can also be distinguished on the basis of the rock type (crystalline versus limestone). Despite the presence of large blocks, we did not take any samples from the moraines of these local moraines, because the quartz-free sedimentary rocks of this area cannot be dated using ¹⁰Be.

4.2 Erstfeld Sackberg

Between Erstfeld and Attinghausen (Fig. 2) a clearly discernible former ice margin on the orographically left side, dipping towards the valley floor, indicates a glacier terminal position in the area of Attinghausen (Attinghausen stage; Hantke, 1980, and references therein; Renner, 1982; Renner in Spillmann et al., 2011). Additional moraines that are attributed to the Attinghausen stage can be found south of the village and less clearly on the other side of the valley in the southern part of Schattdorf. Probably due to a combination of natural erosion and human activity, the terminal moraine has not been preserved. Still, on the basis of the dipping ice margin and the low-lying preserved moraine traces, the location of the former glacier terminus can be well approximated, even if the exact position remains ambiguous. Moraines of a local glacier from the Plattental that were probably deposited in parallel and almost reached the main valley glacier can be found northwest above the clear left-lateral ice margin. We did not find any datable boulders on the moraines of the Attinghausen stage. Instead, we sampled the glacially polished bedrock outcropping in the area of Erstfeld Sackberg, which was dated to 13 200±1910 a. Because this site lies inside the outlines of the Attinghausen stage, the derived age represents a minimum age for this situation. But owing to the relatively high measurement uncertainty of this single exposure age, a conclusive dating or stadial attribution of the Attinghausen stage is not feasible.

Table 2Radiocarbon ages.

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Figure 4Aerial photo of the Reusstal with the outline of the study site Leitschach–Arnisee (Fig. 5). (Photograph © Christine Levy Rothenbühler, 16 October 2016)

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4.3 Leitschach–Arnisee

Approximately 11 km up-valley, the Leitschach meets the Reusstal (Fig. 4). In the lower part of this side valley, distinct moraines of four different glacier stages can be discerned (Fig. 5). The inner two stages representing local glacier advances could not be dated, because suitable boulders were missing. The next older stage, indicating a local glacier terminal position close to the mouth of the side valley, consists of very clear moraines with large boulders. We have dated two samples on the left-lateral moraine: Arnisee 3 (14 600±930 a) and Arnisee 4 (13 350±650 a). Stratigraphically older, the fourth preserved glacier stage is represented by a left-lateral moraine bending in the direction of the Reusstal, which implies a connection to the former Reussgletscher filling up the main valley to this altitude. The corresponding frontal position of the Reussgletscher cannot be conclusively inferred, but a connection with the Attinghausen stage appears to be the most plausible scenario. The two dated samples Arnisee 1 and Arnisee 2 produced exposure ages of 13 250±700 a and 11 700±720 a. In view of their morphostratigraphic position, these dates should be older than the samples Arnisee 3 and 4. The obtained exposure ages indicate a logically impossible age inversion and do not reflect the local stratigraphic sequence. Presumably this moraine degraded over time, so that the boulders were exhumed after the deposition of the moraine, which leads to an underestimation of the real moraine age. This interpretation is supported by the radiocarbon date from the peat bog east of the Arnisee: the transition from inorganic sediment to peat and therefore the starting point for the development of the peat bog was dated to 15 108–14 416 cal a BP. The starting point of the bog development can therefore most likely be attributed to the Bølling interstadial (14.6–14.0 ka) (Table 2). This date represents a minimum age for the disappearance of the glacier from this location. The moraine of the local Leitschachfirn joining the Reussgletscher in the main valley thus has to be older. Accordingly, a pre-Bølling–Allerød age can be inferred for the corresponding glacier terminal position in the Reusstal, presumably lying in the area of Attinghausen. This interpretation would also be affirmed by the age of the sample Erstfeld Sackberg (within its high measurement uncertainties).

Figure 5Map and photos of the moraines and the bog at the Leitschach–Arnisee sample sites. The camera symbols show the perspective seen in the photographs. Reproduced by permission of swisstopo (BA18043).

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Figure 6Map and photos of the moraines at the Fellital sample site. The camera symbols show the perspective seen in the photographs. Reproduced by permission of swisstopo (BA18043).

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4.4 Fellital

North of Wassen, where the Fellital meets the Reusstal, three pronounced right-lateral moraines and one left-lateral moraine document a local glacier advance to a nearby terminal position (Fig. 6). Remnants of a left-lateral moraine belonging to this advance can also be found up-valley at the site Felliberg. To constrain the age of this glacier outline, we dated two boulders. Sample Fellital 1 yielded an age of 11 240±610 a. Fellital 2 was dated to 6700±500 a. In view of the stratigraphic position inside the Reusstal, sample Fellital 1 could represent a reasonable deposition age. Within the measurement uncertainties this age can be attributed to the late phase of the YD. A local glacier terminal position in the nearby village of Göschenen has recently been dated to the YD (Boxleitner et al., 2017); a correlation of these advances therefore appears possible. Nevertheless, on the basis of just one age we cannot conclusively prove whether the obtained age reflects the true age of the moraine deposition. Considering the stratigraphic position, the moraine could also be older.

The age of sample Fellital 2 is clearly too young. Most likely, this boulder was still covered by moraine material during the early phase of the Holocene. The very steep inner side of the moraine seems to be the result of the active erosion of the Fellibach (Felli creek). Over-steepening might have led to lateral erosion and finally to a lowering and shift of what now appears to be the moraine crest. The boulder was, thus, most probably exhumed.